Contemporary spray dampening control systems such as used in rotary printing presses utilize voltage as the source of electromagnetic energy to drive valves. For sake of simplicity, only a discussion of a unipolar valve is given. Bipolar counterpart valves have an additional stage which is conceptually identical, but opposite in polarity. For a description of unipolar versus bipolar operation, refer to our concurrently filed application entitled “Bistable Converter in a Spray Dampening System.”
Referring to FIG. 1, the most basic, and the most prevalent form of current technology consists of a load 10 comprising the solenoid coil of a valve, electrically connected in series with one or more transistors 12 acting as a switches. Opposing voltage potentials V1, V2 are applied to opposite ends of the circuit, 16, 18. Included in the schematic shown in FIG. 1 is parasitic impedance 14 introduced by connectors, long wire length, and other various electrical anomalies found in an actual application resulting in a voltage drop Vp. When the valve is to be engaged, the transistor switch 12 is closed, applying the voltage [V2-V1-VP] across load 10. V1 is the ground voltage and V2 is the positive voltage and VP is the loss of voltage across impedance 14, which is inherent in all voltage control systems. The voltage across the load allows current to flow through the load according to Ohms Law, V=IR, where load 10 provides the resistance R. This current 12 generates a magnetic field, which actuates the valve. When the transistor switch 12 is opened, the flow of current stops, and the valve becomes de-energized.
Three types of voltage control systems have been offered in the prior art. They are conventional, step voltage, and pulse width modulated (PWM) voltage control techniques.
Conventional
The most widespread application is one where the solenoid device is activated by closing a switch. The solenoid device is deactivated by opening the switch, thus removing the voltage placed across the solenoid. In a unipolar solenoid valve, a mechanical device such as a spring returns the valve to its normal, de-energized state.
Step Voltage
A solenoid valve requires far more energy to open, than to remain open. There have been designs which deliver a higher “open” voltage to move the mechanical actuator to an “open” position (see FIG. 3). This is followed by a less intense “hold” voltage to ensure that the actuator remains “open” for the desired interval.
Pulse Width Modulated (PWM) Voltage
For the same purpose as the “step voltage” system shown in FIG. 3, the PWM system uses full voltage for the “open” interval. Afterward the full voltage is switched on and off at intervals, to create a lower average voltage for the “hold” interval, as shown in FIG. 4.